Corals have a calcium carbonate skeleton which is formed and maintained by interaction with the ocean.

As atmospheric carbon dioxide increases, more of the gas is dissolved in the ocean leading to a rise in bicarbonate ions, a fall in carbonate ions and an increase in acidity.

Most studies suggest that this will make it harder for coral to maintain their calcium carbonate skeleton.

But, says Comeau, there are still a lot questions about how exactly corals maintain their

skeleton.

"We don't really know if corals use the carbonate ion to calcify or if they use the biocarbonate ion," he says.

Tank study

To answer this question, Comeau and colleagues looked at a species of coral called Porites rus, collected from their research site in French Polynesia.

They collected samples from different colonies and put them in tanks where they studied how well corals calcified under nine different experimental conditions.

By adding or removing CO2, or by adding acids or bases, Comeau and colleagues were able to manipulate the amount of carbonate and bicarbonate ions present in the tanks.

In addition, the corals were subjected to 12 hours of light and dark to simulate day and night in the field.

Comeau and colleagues found that during night time, the corals relied on carbonate ions to build their skeletons.

However, during the day it was a different story.

"Under light conditions they use both carbonate and bicarbonate ions," says Comeau.

He says most of the studies that look at the effect of ocean acidification focus on a chemical state that is equivalent to the carbonate ion concentration.

The fact that at least one species of coral uses bicarbonate ions bodes well, says Comeau.

"Hopefully this will limit the effect of ocean acidification," he says.

"Maybe some species like this that can use bicarbonate ions in the day time will not likely be impacted highly by ocean acidification."

Explaining anomalies

Australian marine biologist, Dr Ross Jeffree who is an adjunct professor at the University of Technology, Sydney, who has previously collaborated with Comea welcomes the study.

He says a few studies have shown corals to be resistant to ocean acidification and these have been puzzling for marine scientists.

"The most important thing about the study is that it focussed on clarifying the mechanisms of calcification and helps explain anomalies in the scientific database at the moment," says Jeffree.

But, he says the study is only short term and did not include temperature as a variable, which is known to reduce the resistance of marine organisms to ocean acidification.

"This is a major shortcoming of this study," he says.

Comeau and colleagues also investigated the impact of different chemstry on an algae called Hydrolithon onkodes, which acts as a cement on coral reefs, and provide a surface that coral larvae like to settle on.

They found the algae uses both carbonate and bicarbonate ions during night and day, but is more sensitive, than the coral, to a change in the ions.

"They need a big increase in bicarbonate to compensate for a small decrease in carbonate," says Comeau.